As it is known, numerical-control gantry milling machines usually comprise: a workpiece platform, on which the workpiece to be machined is to be placed; a straight load-bearing beam with a high-rigidity structure, which extends horizontally above the workpiece platform, with the two axial ends resting in sliding manner on two lateral shoulders rising from the basement, on opposite sides of the workpiece platform, so as to allow the load-bearing beam to move horizontally above the workpiece platform in a horizontal direction perpendicular to the longitudinal axis of the beam; a spindle-holding carriage which is fixed in axially sliding manner to a lateral flank of the load-bearing beam, so as to be able to move along the beam parallel to the beam longitudinal axis; a spindle-holding shaft which is fixed in vertical position on the spindle-holding carriage with the capability of moving relative to the spindle-holding carriage parallel to its longitudinal axis, i.e. in vertical direction, so as to be able to vary the distance from the beneath-located workpiece platform; and an electric spindle which is housed in vertical position inside the spindle-holding shaft, with the tool-holding head projecting from the lower end of the shaft, so as to be able to reach the workpiece to be machined stationary on the workpiece platform beneath.
The workpiece(s) to be machined, on the other hand, is/are usually fixed on the workpiece platform with the aid of specific intermediate holding structures.
Unfortunately, these intermediate holding structures are made of metal material and this can lead to a significant reduction in the degree of precision of the machining carried out on the workpiece.
The heat produced by the material-removing machining, in fact, is transmitted to the holding structures which expand in an unforeseeable manner, thus moving the workpiece to be machined relative to the initial reference position, even by some tenths of millimeters. Movement that unfortunately results in an error in the positioning of the tool, with all the problems that this entails.
The numerical-control gantry milling machines, in fact, move the tool in space using a reference system which is based on the position of the workpiece to be machined immediately before the beginning of the machining, not on the position assumed after the thermal expansions which the holding structures are subjected to during the machining.
In order to solve this problem, during the machining, the workpiece to be machined is usually cooled down with jets of cooling liquid that remove a large part of the heat produced by the machining.
Despite this expedient, usually the numerical-control gantry milling machines are not capable of reaching machining precisions exceeding the hundredth of millimeter.
Experimental tests, in fact, have shown that, despite the use of jets of cooling liquid, the machining of the workpiece creates, on the workpiece platform and on the single holding structures, anomalous temperature distributions which cause local thermal expansions that temporarily alter, in an unforeseeable manner, the geometry of the single components, thus moving the workpiece to be machined relative to the initial reference position.